As a property owner, you can help control the spread of Dutch elm disease by checking your property and neighborhood for dead or dying elm trees and by checking your woodpile for elm wood with intact bark. If you are not sure if your woodpile contains elm wood or you suspect there are dead or dying elm trees in your neighborhood, please contact the
What is Dutch Elm Disease?
Dutch elm disease is the most destructive disease of elms in North America. It was first identified in the Netherlands and northern France in 1919. This disease spread rapidly throughout Europe and by 1934, it was found in most European countries. The first infections in North America were identified in Ohio in 1930. Dutch elm disease was introduced to the United States from Europe in logs which contained both the fungus and the smaller European elm bark beetle. It has now spread throughout North America and has been reported in all states except those in the desert Southwest. Minnesota's first case of Dutch elm disease was detected in St. Paul in 1961 and has been reported in all but a few of Minnesota's 87 counties.
Dutch elm disease is caused by an aggressive fungus (Ophiostoma novo-ulmi), which invades and grows in the water conducting vessels (xylem) located just beneath the bark. The infected elm responds to the presence of the fungus by producing gums and growths (tyloses) that are designed to block the fungus. Unfortunately, the tree is unable to contain the fast moving fungus. To make matters worse, these gums and growths plug the water conducting vessels and prevent water uptake. This causes the tree to wilt and die.
The first symptom of Dutch elm disease is the wilting of leaves on one or a few branches in the upper canopy of the tree. The next stage is termed flagging as the affected leaves first turn yellow or brown and then shrivel. These leaves may fall from the tree prematurely. Sometimes the leaves dry out very quickly, turning dull green or brown, and can remain hanging on the branches for weeks or months before falling. As the disease progresses, often at a rapid rate, the affected branches die and more of the tree becomes infected.
How Does Dutch Elm Disease Spread?
Elm Bark Beetles
The Dutch elm disease fungus is spread primarily by the European and the native elm bark beetles, both of which are about 1/8 inch long (3 mm). The European elm bark beetle (Scolytus multistriatus) is the primary vector (disease carrier) in the southern portion of Minnesota, including the Twin Cities Metropolitan area, while the native elm bark beetle (Hylurgopinus rufipes) is the primary vector in the northern region of the state.
Adult elm bark beetles lay their eggs under the bark of recently dead or dying elm trees in brood galleries (tunnels in the inner bark where the eggs are laid) constructed by the female beetles. Eggs laid along the brood galleries hatch and the larvae feed, producing their own galleries at right angles to the original egg laying galleries.
If the breeding takes place in diseased wood, the Dutch elm disease fungus spores grow in the beetle brood galleries. These sticky spores adhere to the bodies of the adult beetles before they emerge from the tree. The beetles then fly to a live elm tree and bore into small 2-4 year old branch crotches in the upper canopy to feed. The germinating fungus spores enter the water conducting vessels through the beetle feeding wounds. Another elm tree is then infected with the disease. After feeding, the female adult beetles will seek out recently dead or dying elm wood in order to breed and lay eggs, producing yet another generation of beetles and repeating the cycle. There are generally 2-3 generations of bark beetles per year. Once an elm tree is infected, the disease can spread underground to nearby healthy elms through grafted root systems.
Managing the Spread of Dutch Elm Disease
A Dutch elm disease control program will be successful only if organized on a community-wide basis with the involvement and cooperation of property owners. The prompt removal and proper disposal of all dead and dying elm wood with intact bark is the single most important tool in combating Dutch elm disease. Sanitation consists of the following disease management practices:
Annual Inspections
During the growing season, frequent
Immediate Removal of Infected Elms
The immediate removal of diseased elms during the growing season will greatly reduce the spread of the disease. Also, the immediate removal of all dead and dying elm wood with intact bark (including elm stumps), and pruning of dead elm branches will eliminate potential beetle breeding sites. This includes limbs hanging on trees that may have been damaged by storms the previous season, trees that are very old or weakened by other diseases or insect pests, and fresh elm firewood. These efforts will greatly reduce the number of elm bark beetles and slow the spread of the disease.
Proper Disposal of all Elm Wood
Any dead, dying, or weak elm trees or elm wood with bark firmly attached can serve as a breeding site for bark beetles. Therefore, all elm wood must be properly disposed of by debarking, chipping, burning or burying in order to guarantee that the elm wood is unsuitable for the beetles. Debarking of the elm wood will allow it to be used for firewood. Chipped elm wood can be used for mulch, animal bedding, and trails.
Elm Firewood Control
Both species of elm bark beetles require elm wood with intact bark to survive and breed successfully. Elm firewood with intact bark may play a major role in the overwintering survival of elm bark beetles. Since the interior of a firewood pile offers a protected environment, beetle survival may be higher than in standing trees or fallen logs. Therefore, elm firewood is an especially good breeding place for the bark beetles. As many as 1800 new adult beetles have been found in a single fireplace-size log (4 x 22 inches). A complete dead elm tree left to stand could produce tens of thousands of beetles. If the elm wood is infested with the fungus, each emerging beetle carrying the fungus spores could then inoculate healthy trees during their feeding activities.
Root Graft Disruption
Elm trees that are growing close together (within 50 feet) generally will form a common root system as roots entangle and graft together. If one elm becomes infected with the disease, the fungus will be transmitted through the root system into adjoining healthy elms. Root graft cutting is accomplished by using a vibratory plow with a 5-foot blade. The infected tree should be removed after root graft disruption.
Chemical Treatment
Systemic fungicides, when properly injected into elms inoculated by beetles, can save elm trees in the early stages of the disease when less than 10 percent of the crown has wilted. The chemical is injected into the root flares below grade via the macro injection process. These fungicides should be used only by trained arborists. There are several in the Twin Cities metropolitan area that offer this service. They should be contacted for more detailed information regarding the chemical treatment of diseased elms.
Licensed Tree Contractors
Looking for a reputable tree care company to remove diseased elm trees? View the list of licensed tree contractors.
